In the normal combustion of a fuel-air mixture in an internal combustion engine, the fuel-air mixture is ignited by an ignition spark and then burns in a controlled and progressive manner as the flame front advances through the compressed fuel-air mixture in the cylinder chamber. However, an undesired auto-ignition and uncontrolled explosion of the as-yet un-burned fuel-air mixture, commonly known as "knocking", "pinging", or "detonation", can also occur. Such knocking is generally to be avoided because it causes intense pressure waves oscillating in the cylinder, which cause a vibration of the engine components and a resultant audible knocking noise. Ultimately, the intense knocking forces can damage or destroy the engine. Nonetheless, under at least some operating conditions, an engine can achieve its maximum power output and efficiency by operating directly at the limit or boundary of knocking conditions. Thus, an engine controller, such as the conventionally known electronic control unit (ECU), aims to operate the engine as close as possible to the knocking limit without actually causing knocking. If knocking does occur, then corrective measures are taken, for example the ignition timing is slowly retarded, i.e. adjusted in a direction toward the top dead-center position of the piston.
Knocking combustion is generally characterized by pressure oscillations having a frequency in the range from 5 to 20 kHz, taking place in a time interval following the maximum cylinder pressure, i.e. maximum compression. The knocking combustion can be detected by measuring and evaluating changes in the ionic current that flows within the cylinder combustion chamber. This ionic current can be sensed by a suitable sensor arranged in the cylinder, for example a spark plug may be used as an ionic current sensor. However, the ionic current signal already exhibits a first maximum signal level as well as oscillations of the ionic current at the time when the flame front advances and spreads through the fuel-air mixture in the cylinder. This maximum signal level and oscillations in the ionic current can be misevaluated to result in an erroneous detection of knocking when knocking has not actually occurred, because these signal oscillations are caused by turbulence in the cylinder and not by knocking.
Knock recognition methods using an evaluation of the ionic current signal are known in the prior art, but all suffer the disadvantage that they are very sensitive to interference and often result in false detections of knocking. A major cause of such problems in the prior art is that variations in the composition of the fuel, and especially with regard to any metallic components in the fuel, have a strong influence on the ionic current signal over time. Also, the overall signal level or amplitude of the ionic current signal is substantially increased or even multiplied several times, if even a relatively small amount of heavy metal impurities or other metallic components are present in the fuel. This can occur, for example, due to a contaminated production of the fuel or due to accidental fueling of the engine with leaded fuel. Since these variations in the fuel and resulting variations in the ionic current signal are not taken into account according to the prior art, the known methods and circuit arrangements can erroneously determine that knocking is occurring even when a proper combustion without knocking is actually taking place.